Heat Recovery Steam Generator (HRSG)

Heat Recovery Steam Generator (HRSG) is a boiler that utilizes heat energy of residual exhaust gases from gas turbine unit to heat water and convert it into steam, and steam is then used to drive steam turbines. In general, Heat Recovery Steam Generator (HRSG) boiler is not equipped with burners and do not consume fuel, so there is not radiation heat transfer process. The heat transfer process that occurs only convection and conduction from exhaust gas of gas turbine into water to be processed into steam through heating elements inside HRSG boiler room.
Figure 1:  Combined Cycle Power Plant (CCPP)

Heat Recovery Steam Generator (HRSG) boiler is very useful to improve efficiency of fuel that is used in gas turbine unit, which will further drive steam turbine unit. Power generation systems that take advantage of this process is called Combined Cycle Power Plant (CCPP). Combined Cycle Power Plant (CCPP) is a combined gas turbine Brayton cycle and steam turbine Rankine cycle. HRSG boiler is part of the Rankine cycle.

TS diagram illustrates the overall process of Combined Cycle Power Plant (CCPP) which is shown in Figure 2. Diagram I expresses Brayton cycle for gas turbine and diagram II states Rankine cycle for steam turbine.
Figure 2: Diagram CCPP and HRSG Single Pressure
Figure 3: Diagram T-S Combined Cycle Power Plant (CCPP)

Steam production capacity that can be generated by Heat Recovery Steam Generator (HRSG)depend on the capacity of heat energy that is still contained in exhaust gas from gas turbine unit, which means depend on the load of gas turbine unit. Basically, gas turbines operate at fixed rotation, the flow of incoming air compressor also fixed but it may occurs changes in load of turbine although the fuel flow is constant, so that the temperature of exhaust gas  will also changes following the change of gas turbine load.

Exhaust gas temperature from gas turbine unit can maintained constant by regulating the opening of Inlet Guide Vane (IGV) to regulate the flow rate of air into compressor in which the exhaust gas temperature act as feedback.
Figure 4: Flow Diagram of Heat Recovery Steam Generator (HRSG)

Some Heat Recovery Steam Generator (HRSG) boilers has some components and can be equipped with additional burner to increase steam production capacity and some production of steam can be used for heating purposes of other applications (cogeneration). With this additional combustion,  the stability of steam production HRSG can be maintained. Furthermore the stability of steam turbine that uses steam can be maintained too, although the gas turbine load changing and also temperature exhaust gas of gas turbine (air flow into the compressor) should not be kept constant (IGV setting is not required).

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